quadrupole mass filter and counted with a Daly detector. The signal intensity of the mass-selected fragment with respect to its time-of-arrival is recorded with a ...
The influence of local electronic character and nonadiabaticity in the photodissociation of nitric acid at 193 nm T. L. Myers, N. R. Forde, B. Hu, D. C. Kitchen, and L. J. Butler The James Franck Institute and Department of Chemistry, The University of Chicago, Chicago, Illinois 60637
~Received 10 February 1997; accepted 8 July 1997! The dissociation of nitric acid upon p nb,O→ p * NO2 excitation at 193 nm has been studied in a crossed laser-molecular beam apparatus. The primary reaction channels are OH1NO2 and O1HONO. We measure the branching ratio between these two competing processes and determine ~OH1NO2!/~O1HONO!50.5060.05. Our experiments provide evidence of a minor O1HONO pathway, which we assign to O( 3 P) and HONO in its lowest triplet state. The dominant pathway correlates to O( 1 D)1HONO(X 1 A 8 ). The translational energy distributions reveal two distinct pathways for the OH1NO2 channel. One pathway produces stable NO2 fragments in the 1 2 B 2 electronic state. The second pathway produces unstable NO2 fragments which undergo secondary dissociation to NO1O. We examine the influence of nonadiabaticity along the OH1NO2 reaction coordinate in order to explain the significant branching to this other channel. Finally, we introduce a new method for generating correlation diagrams for systems with electronic transitions localized on one moiety, in which we restrict the changes allowed in remote molecular orbitals along the reaction coordinate. Analysis of previously measured X1NO2 photofragment pathways in nitromethane and methyl nitrate provides further support for using a restricted correlation diagram to predict the adiabatic and nonadiabatic product channels. © 1997 American Institute of Physics. @S0021-9606~97!01638-3#
I. INTRODUCTION
While the bulk of this paper seeks to investigate the specific competing adiabatic and nonadiabatic dissociation channels of nitric acid at 193 nm, two results in the paper address broader issues. First, the adiabatic OH1NO2(1 2 B 2 ) reaction pathway, which is one of the two OH1NO2 channels experimentally observed, belongs to a general class of reactions where the individual orbital symmetry is not conserved along the reaction coordinate. Thus, this system provides the opportunity to test whether such reactions are particularly susceptible to electronically nonadiabatic effects. Second, the analysis of our results on nitric acid suggests an approach to determine what product channels are inaccessible when nonadiabatic effects play an important role. We find in nitric acid that product channels which require the OH radical orbital to change symmetry from a 8 in the reactant region to a 9 in the product region, with a complementary change in the occupied NO2 orbitals necessary to maintain the overall state symmetry, simply are not observed experimentally. Such a change in the electronic wave function during reaction is evidently even more difficult for a molecule to accomplish than a switch in the individual orbital symmetry when both sets of orbitals are localized on the same moiety. Thus, we introduce a ‘‘restricted adiabatic’’ correlation diagram that disallows correlations to product channels when such drastic changes in the electronic wave function are required. In the specific, but not narrow, class of systems in which the relevant molecular orbitals are effectively localized in both the reactants and products, such restricted correlation diagrams should prove useful in identifyJ. Chem. Phys. 107 (14), 8 October 1997
ing which reaction channels are viable ones. The potential significance to any reactive system with electronically localized functional groups is clear. Many research groups have investigated the photodissociation of nitric acid in the ultraviolet region due to its importance in atmospheric chemistry. Although these studies indicate the major primary products from the photodissociation of nitric acid at 193 nm are OH1NO2 and O1HONO, 1–3 the quantum yields of these products remain disputed. Furthermore, the role of electronic nonadiabaticity in the dissociation dynamics of the competing channels and the identity of the NO2 products have not been elucidated. We begin with a brief review of the relevant experimental and theoretical work on nitric acid before presenting our goals for this paper. The broad and structureless ultraviolet absorption spectrum4 for nitric acid results from three transitions involving orbitals localized on the NO2 moiety. The intense absorption band with a maximum at 190 nm ( s 510217 cm2) involves a p nb,O→ p * NO2 transition to the 2 1 A 8 electronic state. At wavelengths longer than 200 nm in the absorption spectrum, the major primary process is production of OH1NO2 ~Ref. 5!, and there have been several recent detailed studies of the OH photofragment angular distributions, lambda doublet ratios, and v – J correlations,6–9 as well as analysis of the fluorescence from the NO2 product.10,11 Excitation to the 2 1 A 8 excited state for nitric acid leads to both OH1NO2 and O1HONO products. Although the quantum yields for these products have been investigated, conflicting measurements have been reported. Ravishankara and co-workers1 detect OH products via pulsed laser-induced
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© 1997 American Institute of Physics
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fluorescence and report F(OH)50.3360.06. They also use atomic resonance fluorescence detection to determine the quantum yields for O@ O( 3 P)1O( 1 D) # 50.8160.13 and O( 3 P)50.5360.13, which differ to give O( 1 D)50.28 60.07, and they report a minor primary dissociation channel producing H1NO3 with a quantum yield of
